The RSPCA Assured label indicates that the farms have been monitored and assessed against the RSPCA welfare standards for pigs.*
The Red Tractor label indicates that the pork has been farmed, processed and packed in the UK. Its animal welfare standards are in line with, and often above the UK legal minimum standard.**
Organic labels such as The Organic Food Federation, Welsh Organic, Soil Association and OF&G require that the animals have been fed an organic diet. There are also strict rules regarding the use of antibiotics and other medicines. Pigs in organic systems have access to the outdoors throughout their lives.
In Scotland, the QMS Specially Selected Pork label indicates the pork is Scottish, and that the farms may have been checked by the Scottish SPCA. Many of the standards reflect basic legislation.
EU legal standards are similar to those in the UK but also allow the use of sow stalls (small, narrow individual pens) for the first four weeks in a sow's pregnancy, which have been illegal in the UK for over 20 years.
The UK legal standard for pigs currently allows pregnant sows to be kept in farrowing crates for several weeks. A solid sleeping area or bedding materials are not required and pigs can legally be castrated without anaesthetic. There's not an official UK pork label and no requirement for welfare checks, but the Union Jack, or the words 'British Pork' are often used on packaging.
Standards comparison accurate as of August 2022
* https://science.rspca.org.uk/sciencegroup/farmanimals/standards/pigs
** Red Tractor introduced enhanced welfare standards for pigs in November 2021, which include requirements for dry sleeping areas, veterinary approval for tail docking and the provision of beneficial enrichment.